the panel on the inside is for running on gas. when u turn it on, it will use gas and if the electric is turned on, both will heat. the reset (i believe) is just a light that flashes when it resets.
as has already been stated, make sure there is water in the tank before u turn on either the electric or gas. the electric element will only last a few seconds with no water around it. open the relief valve (on the hot water heater) and make sure u get water out of it to make sure it is full. don't just go by a flow at the hot water faucets. (if the water heater is still bypassed, u will get water at the faucets with the heater empty).
the probe (anode), if it is abt half or more gone (down to the steal core that will rust) replace it.
the last time i used the gas was when i bought the trailer in 2002. my electric element was burnt out...either it was turned on during the trailer show, during the walk thru, or by me. bottom line, it was turned on with no water in the heater and burnt out.